Image printing apparatus and method of adjusting and correcting main-scanning offset in image printing apparatus

ABSTRACT

This invention is directed to an image printing apparatus which can print images without any halfway offset or obverse/reverse pixel offset at a low cost. An image printing apparatus includes a clock generating section which generates a dot clock as the basis of each pixel forming an image, an image printing section which prints a one-line image in a main scanning direction in accordance with image data with reference to the dot clock output from the clock generating section, and prints a one-page image by repeating in the sub-scanning direction one-line image printing performed in the main scanning direction, and a clock control section which changeably controls the frequency of the dot clock during scanning of one line in the main scanning direction.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Divisional Application of U.S. applicationSer. No. 10/692,268 filed Oct. 23, 2003, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image printing apparatus whichprints a one-line image in the main scanning direction in accordancewith image data with reference to a dot clock as the basis of each pixelforming an image, and also prints a one-page image by repeating in thesub-scanning direction one-line image printing performed in the mainscanning direction.

2. Description of the Prior Art

As an image printing apparatus, an apparatus for printing a one-lineimage in the main scanning direction in accordance with image data andprinting a one-page image by repeating in the sub-scanning directionone-line image printing performed in the main scanning direction isknown.

Image printing in the main scanning direction in accordance with imagedata is performed with reference to a clock signal (to be referred to asa “dot clock” hereinafter in this specification) as the basis of eachpixel to be formed.

For example, in an electrophotographic image printing apparatus, a laserbeam modulated in accordance with image data is scanned in the mainscanning direction, and an image is concurrently formed on an imagecarrier rotating in the sub-scanning direction with the laser beam.

In this case, a laser beam is modulated with image data with referenceto a clock signal (pixel clock) called a dot clock.

The arrangement of a writing unit as the main component of an imageprinting apparatus using a laser beam in this manner will be describedwith reference to FIG. 14.

In a writing unit circuit section 200, a laser diode (LD) 260 emits alaser beam LB on the basis of a laser driving signal LS generated on thebasis of a dot clock.

The laser beam LB from the LD 260 passes through a collimator lens 261and cylindrical lens 262 and is then scanned in the main scanningdirection by a polygon mirror 263.

The laser beam LB scanned by the polygon mirror 263 is adjusted to bescanned at a constant velocity by an fè lens 264. The laser beam furtherpasses through a cylindrical lens 265 to strike a photosensitive drum10.

Sub-scanning operation is then performed by rotating the photosensitivedrum 10 during this main-scanning operation. By repeating main-scanningoperation in the sub-scanning direction, a one-page image is printed.

Note that part of the laser beam LB scanned by the polygon mirror isguided to an index sensor 266 to detect the timing.

In an image printing apparatus designed to print a one-page image byrepeating such image printing operation, performed in the main scanningdirection, in the sub-scanning direction, the main-scanningmagnification (the magnification in the main scanning direction) maydiffer from a desired magnification due to the power characteristics andmount precision of various kinds of lens systems, the planarity andmount precision of various kinds of mirrors, the planarity and mountprecision of a photosensitive drum as an image printing medium (imagecarrier), individual differences thereof, and the like. That is, anoffset may occur in the main scanning direction.

An invention designed to change the frequency of a dot clock to adjustsuch a main-scanning magnification (eliminate an offset in the mainscanning direction) is disclosed in, for example, patent reference 1below.

A plurality of writing units may be provided for the above imageprinting apparatus to allow it to print a color image. In this case, ifoffsets in the main scanning direction occur between the respectivewriting units, a proper color image cannot be printed.

In order to eliminate such offsets in the main scanning direction, thefrequency of a dot clock must be changed for each color. Conventionaltechniques of this type are disclosed in, for example, the following twopatent references:

patent reference 1: Japanese Unexamined Patent Publication No.2000-199868 (page 4; FIG. 7)

patent reference 2: Japanese Unexamined Patent Publication No.2000-202648 (page 13; FIG. 1)

Repeated studies by the present inventors show that the followingproblems arise even though the main-scanning magnification is adjustedto coincide with the desired magnification by the techniques disclosedin the two patent references described above.

Even if the distance between the start and the end in the main scanningdirection is made equal to a desired distance, a halfway point, e.g., anintermediate point between the start and the end, may not alwayscoincide with a desired position.

In other words, in some case, although the two ends, i.e., the start andthe end, coincide with desired positions, a slight offset (halfwayoffset) may have occurred between the two ends.

In the field of offset printing or the like, in particular, not onlyexpansion/contraction between the start and the end but also a pixeloffset at a halfway point (halfway offset) may pose serious problems.

The factors that cause such halfway offsets include the powercharacteristics and mount precision of various kinds of lens systems,the planarity and mount precision of various kinds of mirrors, theplanarity and mount precision of a photosensitive drum as an imageprinting medium (image carrier), individual differences thereof, and thelike. An enormous cost is required to manufacture and mount thesecomponents with high precision without causing any halfway offset.

In printing a color image, if such halfway offsets occur in differentproportions among the respective colors, the offsets appear asnoticeable color misregistration, posing a serious problem.

If the image printing apparatus is an image printing apparatus capableof printing images on the two surfaces of an image recording sheet, evenif the distance between the start and the end in the main scanningdirection is adjusted to a desired value, a user may recognize thedifference between the distances on the two surfaces as an offset.

This is a case wherein an image recording sheet having images printed onits obverse and reverse surfaces is held up against light, an offsetbetween the obverse and reverse surfaces is recognized. Obviously, inthis case, a halfway offset may have occurred.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems in theprior art, and has as its object to print images without any halfwayoffset or obverse/reverse pixel offset at a low cost.

In order to achieve the above object, according to the first aspect ofthe present invention, there is provided an image printing apparatuscomprising a clock generating section which generates a dot clock as abasis of each pixel forming an image, an image printing section whichprints a one-line image in a main scanning direction in accordance withimage data with reference to the dot clock output from the clockgenerating section, and prints a one-page image by repeating in thesub-scanning direction one-line image printing performed in the mainscanning direction, and a clock control section which changeablycontrols a frequency of the dot clock during scanning of one line in themain scanning direction.

According the first aspect, when a one-line image is printed in the mainscanning direction in accordance with image data with reference to a dotclock and a one-page image is printed by repeating in the sub-scanningdirection one-line printing performed in the main scanning direction,the frequency of the dot clock is changeably controlled during scanningof one line in the main scanning direction.

According to the second aspect of the present invention, there isprovided an image printing apparatus, wherein the clock control sectiondescribed in the first aspect controls even the number of pixels of oneline in the main scanning direction in a case in which the frequency ofthe dot clock is changed during scanning of one line to be equal to thatin a case in which the frequency is not changed.

According to the second aspect, even if the frequency of the dot clockis changed during scanning of one line, the number of pixels of one linein the main scanning direction in a case in which the frequency of thedot clock is changed is controlled to that in a case in which thefrequency is not changed.

According to the third embodiment, there is provided an image printingapparatus, wherein the clock control section described in the first orsecond aspect changes the frequency of the dot clock during scanning ofone line in the main scanning direction on the basis of a plurality ofmain-scanning partial offsets in a test image printed by the imageprinting apparatus in accordance with test image data.

According to the third aspect, the frequency of the dot clock is changedduring scanning of one line in the main scanning direction on the basisof a plurality of partial main-scanning offsets in test images printedby the image printing apparatus in accordance with test image data.

According to the fourth aspect of the present invention, there isprovided an image printing apparatus, wherein the image printingapparatus described in the first or second aspect further comprises anoffset setting section which sets at least two offsets between distancesbetween test patterns in test images printed by the image printingapparatus in accordance with test image data having test patternsarranged at least three positions in the main scanning direction andreference distances between comparative test patterns, and the clockcontrol section changes the frequency of the dot clock during scanningof one line in the main scanning direction on the basis of therespective offsets set by the offset setting section.

According to the fourth aspect, at least two offsets are set between thedistances between test patterns in test images printed in accordancewith test image data having test patterns arranged at least threepositions in the main scanning direction and the reference distancesbetween comparative test patterns, and the frequency of the dot clock ischanged during scanning of one line in the main scanning direction onthe basis of the respective set offsets.

According to the fifth aspect of the present invention, there isprovided an image printing apparatus, wherein the clock control sectiondescribed in the fourth aspect changes the frequency of the dot clock toevenly distribute the offsets to the respective pixels between the testpatterns so as to eliminate the offsets.

According to the fifth aspect, offsets are detected between three ormore main-scanning test patterns used for image printing, and thefrequency of the dot clock is changed to evenly distribute the offsetsto the respective pixels so as to eliminate the offsets.

According to the sixth aspect of the present invention, there isprovided an image printing apparatus described in the fourth or fifthaspect, wherein the image printing apparatus comprises a test patterinterval detecting section which computes distances between the testpatterns in the test images by detecting positions of the test patterns,and an offset computing section which computes offsets between thedistances between the test patterns which are computed by the testpattern interval detecting section and the reference distances betweenthe comparative test patterns, and at least two offsets computed by theoffset computing section are set by the offset setting section.

According to the sixth aspect, the positions of test patterns aredetected, and the distances between the test patterns are computed. Theoffsets between the computed distances between the test patterns and thereference distances between comparative test patterns are computed. Atleast two computed offsets are set by the offset setting section.

According to the seventh aspect of the present invention, there isprovided an image printing apparatus described in any one of the fourthto sixth aspects, wherein the reference distances are distances betweenthe test patterns in the test image data.

According to the seventh aspect, at least two offsets are set betweenthe distances between test patterns in test images printed in accordancewith test image data having test patters arranged at least threepositions in the main scanning direction and the reference distancesbetween comparative test patterns, and the distances between the testpatterns in the test image data are used as reference distances when thefrequency of the dot clock is changed during scanning of one line in themain scanning direction on the basis of the respective set offsets.

According to the eighth aspect of the present invention, there isprovided an image printing apparatus, wherein the image printingapparatus described in any one of the fourth to sixth aspect printsimages on two surfaces of an image recording sheet, and the clockcontrol section changes the frequency of the dot clock when an image isprinted on one surface of the image recording sheet or images areprinted on the two surfaces.

According to the eighth aspect, images can be printed on the twosurfaces of an image recording sheet, and the frequency of the dot clockis changed when an image is to be printed on one surface of the imagerecording sheet or images are to be printed on the two surfaces.

According to the ninth aspect of the present invention, there isprovided an image printing apparatus described in the eighth aspect,wherein the reference distances are distances between the test patternsin the test image data or distances between test patterns in test imagedata printed on a reverse surface of an image recording sheet when thefrequency of the dot clock is changed in printing an image on an obversesurface of the image recording sheet, or the reference distances aredistances between the test patterns in the test image data or distancesbetween test patterns in test image data printed on the obverse surfaceof the image recording sheet when the frequency of the dot clock ischanged in printing an image on the reverse surface of the imagerecording sheet.

According to the ninth aspect, at least two offsets are set between thedistances between test patterns in test images printed in accordancewith test image data having test patterns arranged at least threepositions in the main scanning direction and the reference distancesbetween comparative test patterns, and the distances between the testpatterns in the test images or the distances between test image dataprinted on the other surface are used as reference distances when thefrequency of the dot clock is changed during scanning of one line in themain scanning direction on the basis of the respective set offsets.

According to the 10th aspect of the present invention, there is providedan image printing apparatus, wherein the image printing apparatusdescribed in any one of the fourth to ninth aspects prints an image bysuperimposing an image formed in a first color and an image formed in asecond color different from the first color, and the clock controlsection changes the frequency of the dot clock when one or both ofimages in the first and second colors are to be printed.

According to the 10th aspect, when an image is printed on an imagerecording sheet by using at least two colors, the frequency of the dotclock is changed for at least one of the colors.

According to the 11th aspect of the present invention, there is providedan image printing apparatus described in the 10 aspect, wherein thereference distances are distances between the test patterns in the testimage data or distances between test patterns in test image data printedin the second color when the frequency of the dot clock is changed inprinting an image in the first color, or the reference distances aredistances between the test patterns in the test image data or distancesbetween test patterns in test image data printed in the first color whenthe frequency of the dot clock is changed in printing an image in thesecond color.

According to the 11th aspect, at least two offsets are set between thedistances between test patterns in test images printed in accordancewith test image data having test patterns arranged at least threepositions in the main scanning direction and the reference distancesbetween comparative test patterns, and the distances between the testpatterns in the test images or the distances between test image dataprinted in the other color are used as reference distances when thefrequency of the dot clock is changed during scanning of one line in themain scanning direction on the basis of the respective set offsets.

According to the 12th aspect of the present invention, there is providedan image printing apparatus described in any one of the first to 11thaspects, wherein the clock generating section comprises a fundamentalclock generating section which generates a fundamental clock having apredetermined frequency, and a frequency changing section which canchange the frequency of the fundamental clock generated by thefundamental clock generating section, and the clock control sectionchanges a frequency of a fundamental clock output from the frequencychanging section during scanning of one line in the main scanningdirection.

According to the 12th aspect, the clock control section performs controlto change the frequency of the fundamental clock output from thefrequency changing section during scanning of one line in the mainscanning direction.

According to the 13th aspect of the present invention, there is providedan image printing apparatus described in the 12th aspect, wherein thefrequency changing section has a delay line which generates delay clocksby digitally delaying the fundamental clock, and the clock controlsection changes the frequency of the dot clock during scanning of oneline in the main scanning direction by selecting a predetermined delayclock from the delay line.

According to the 13th aspect, the frequency changing section digitallydelays the fundamental clock by the delay line, and the clock controlsection changes the frequency of the dot clock during scanning of oneline in the main scanning direction by selecting a predetermined delayclock from the delay line.

According to the 14th aspect of the present invention, there is provideda main-scanning offset adjusting method for an image printing apparatus,which adjusts a main-scanning offset in the image printing apparatuswhich generates a dot clock as a basis of each pixel forming an image,prints a one-line image in a main scanning direction in accordance withimage data with reference to the dot clock, and prints a one-page imageby repeating in a sub-scanning direction one-line printing, performed inthe main scanning direction, comprising the steps of printing testimages in accordance with test image data having test patterns arrangedat least three positions in the main scanning direction, obtaining atleast two offsets between distances between the test patterns in thetest images and reference distances between comparative test patterns,and setting the respective offsets such that the frequency of the dotclock can be changed during scanning of one line in the main scanningdirection on the basis of the respective offsets.

According to the 14th aspect, in main-scanning offset adjustment, testimages are printed in accordance with test image data having testpatterns arranged at least three positions in the main scanningdirection, and at least two offsets are obtained between the distancesbetween the test patterns in the test images and the reference distancesbetween comparative test patterns. The respective offsets are set suchthat the frequency of the dot clock can be changed during scanning ofone line in the main scanning direction on the basis of the respectiveoffsets.

According to the 15th aspect, there is provided a main-scanning offsetcorrecting method for an image printing apparatus, which corrects amain-scanning offset in the image printing apparatus which generates adot clock as a basis of each pixel forming an image, prints a one-lineimage in a main scanning direction in accordance with image data withreference to the dot clock, and prints a one-page image by repeating ina sub-scanning direction one-line printing, performed in the mainscanning direction, comprising the steps of setting at least twooffsets, before image printing, between distances between test patternsin test images printed in accordance with test image data having testpatterns arranged at least three positions in the main scanningdirection and reference distances between comparative test patterns, andchanging the frequency of the dot clock during scanning of one line inthe main scanning direction on the basis of the respective offsets setin the offset setting step during image printing.

According to the 15th aspect, in main-scanning offset correction, beforeimage printing, at least two offsets are set between the distancesbetween test patterns in test images printed in accordance with testimage data having test patterns arranged at least three positions in themain scanning direction and the reference distances between comparativetest patterns, and the frequency of the dot clock is changed duringscanning of one line in the main scanning direction during imageprinting operation.

According to the 16th aspect of the present invention, there is providedan image printing apparatus including a clock generating section whichgenerates a dot clock as a basis of each pixel forming an image, and animage printing section which prints a one-line image in a main scanningdirection in accordance with image data with reference to the dot clockoutput from the clock generating section, and printing in thesub-scanning direction a one-page image by repeating one-line imageprinting performed in the main scanning direction, comprising a controlsection which controls image printing in the main scanning directionsuch that distances between test patterns in reference images which arearranged at least three positions in the main scanning directioncoincide with distances between test images printed in accordance withtest image data corresponding to the reference images.

According to the 16th aspect, in image printing, image printing in themain scanning direction is controlled such that the distances betweentest patterns in reference images which are arranged at least threepositions in the main scanning direction coincide with distances betweentest images printed in accordance with test image data corresponding tothe reference images.

According to the 17th aspect of the present invention, there is providedan image printing apparatus, wherein the control section described inthe 16th aspect comprises a clock control section which changeablycontrols the frequency of the dot clock during scanning of one line.

According to the 17th aspect, in image printing, image printing in themain scanning direction is controlled during scanning of one line suchthat the distances between test patterns in reference images which arearranged at least three positions in the main scanning directioncoincide with distances between test images printed in accordance withtest image data corresponding to the reference images.

According to the 18th aspect of the present invention, there is providedan image printing apparatus for printing images on two surfaces of animage recording sheet, which includes a clock generating section whichgenerates a dot clock as a basis of each pixel forming an image, and animage printing section which prints a one-line image in a main scanningdirection in accordance with image data with reference to the dot clockoutput from the clock generating section, and prints a one-page image byrepeating in the sub-scanning direction one-line image printing,performed in the main scanning direction, wherein the frequency of thedot clock can be set to different frequencies depending on whether animage is to be printed on an obverse surface or reverse surface of animage recording sheet.

According to the 18th aspect, when a one-line image is printed in a mainscanning direction in accordance with image data with reference to a dotclock, and a one-page image is printed on each of the two surfaces of animage recording sheet by repeating in the sub-scanning directionone-line image printing performed in the main scanning direction, thefrequency of the dot clock can be set to different frequencies dependingon whether an image is to be printed on the obverse or reverse surfaceof the image recording sheet.

According to the 19th aspect of the present invention, there is providedan image printing apparatus including a clock generating section whichgenerates a dot clock as a basis of each pixel forming an image, a tonerimage forming section which includes a writing section which forms aone-line image in a main scanning direction in accordance with imagedata with reference to the dot clock output from the clock generatingsection, and forms a one-page image by repeating in the sub-scanningdirection one-line image printing performed in the main scanningdirection, the toner image forming section which forms a toner image onone surface of an image recording sheet, and a fixing section whichfixes the toner image formed by the toner image forming section on theimage recording sheet, the image printing apparatus printing images ontwo surfaces of the image recording sheet by causing the toner imageforming section to form a toner image on a reverse surface of the imagerecording sheet having the toner image formed on one surface and causingthe fixing section to fix the image, wherein the frequency of the dotclock can be set to different frequencies depending on whether an imageis to be printed on an obverse surface or reverse surface of an imagerecording sheet.

According to the 19th aspect, when a one-line image is printed in a mainscanning direction in accordance with image data with reference to a dotclock, and a one-page image is printed on each of the two surfaces of animage recording sheet by repeating in the sub-scanning directionone-line image printing performed in the main scanning direction, thetoner image on the reverse surface is fixed after the toner image formedon the obverse surface is fixed. The frequency of the dot clock can beset to different frequencies depending on whether an image is to beprinted on the obverse or reverse surface of the image recording sheet.

According to the 20th aspect of the present invention, there is providedan image printing apparatus described in the 19th aspect, wherein thefrequency of the dot clock can be set to a given frequency when an imageis to be printed on at least one of obverse and reverse surfaces of animage recording sheet.

According to the present invention, when a one-line image is printed ina main scanning direction in accordance with image data with referenceto a dot clock, and a one-page image is printed on each of the twosurfaces of an image recording sheet by repeating in the sub-scanningdirection one-line image printing performed in the main scanningdirection, the toner image on the reverse surface is fixed after thetoner image formed on the obverse surface is fixed. When an image is tobe printed on at least one of the obverse and reverse surfaces of theimage recording sheet, the frequency of the dot clock can be set todifferent frequencies depending on whether an image is to be printed onthe obverse or reverse surface of the image recording sheet.

As is obvious from the above aspects, according to the presentinvention, the following effects can be obtained.

(1) In the first aspect, since the frequency of the dot clock can bechanged even during scanning of one line, not only the starts and endsof lines can be aligned, but also a pixel offset (halfway offset) in anintermediate portion can be adjusted. Therefore, an image can be printedwithout any halfway offset at a low cost.

(2) In the second aspect, since the frequency of the dot clock can bechanged even during scanning of one line, and the number of pixels ofone line in the main scanning direction is controlled to a predeterminednumber, not only the starts and ends of lines can be aligned, but also apixel offset (halfway offset) in an intermediate portion can beadjusted. Therefore, an image can be printed without any halfway offsetat a low cost.

(3) In the third aspect, since the frequency of the dot clock is changedduring scanning of one line in the main scanning direction on the basisof the partial offsets detected by printing test images, a pixel offset(halfway offset) in an intermediate portion can be adjusted. Therefore,an image without any halfway offset can be printed at a low cost.

(4) In the fourth aspect, since offset correction is executed bydetecting offsets (error information) at three or more positions in themain scanning direction, not only the starts and ends of lines can bealigned, but also a pixel offset (halfway offset) in an intermediateportion can be properly adjusted.

(5) In the fifth aspect, pieces of error information are detected atthree or more positions in the main scanning direction, and dot clocksare set in accordance with this detection to evenly distribute thepieces of error information to the respective pixels between therespective test patterns. This makes it possible to properly adjust apixel offset in an intermediate portion as well as aligning the startsand ends of lines.

(6) In the sixth aspect, the distances between test patterns arecomputed in advance, and offset correction is executed by detectingoffsets (error information) at three or more positions in the mainscanning direction. This makes it possible to properly adjust a pixeloffset (halfway offset) in an intermediate portion as well as aligningthe starts and ends of lines.

(7) In the seventh aspect, offset correction is executed by detectingoffsets (error information) at three or more positions in the mainscanning direction using the distances between test patterns in testimage data as reference distances. This makes it possible to properlyadjust a pixel offset (error information) in an intermediate portion aswell as aligning the starts and ends of lines.

(8) In the eighth aspect, in the image printing apparatus capable ofprinting images on the two surfaces of an image recording sheet, sincethe frequency of a dot clock is changed, a pixel offset (halfway offset)in an intermediate portion can be adjusted. In addition, an offsetbetween the two surfaces can be adjusted.

(9) In the ninth aspect, in double-side image printing, offsetcorrection is executed by detecting offsets (error information) at threeor more positions in the main scanning direction using the distancesbetween test patterns in test image data or the distances between testpatterns printed on the other surface as reference distances. This makesit possible to properly adjust a pixel offset (error information) in anintermediate portion as well as aligning the starts and ends of lines.In addition, an obverse/reverse pixel offset can also be properlyadjusted.

(10) In the 10th aspect, when images are to be printed in at least twocolors on an image recording sheet, the frequency of a dot clock ischanged for at least one of the colors.

In the image printing apparatus capable of printing an image in aplurality of colors, a pixel offset between the respective colors can beadjusted by changing the frequency of the dot clock for any one of thecolors. In addition, a pixel offset between the respective colors on thetwo surfaces can also be adjusted.

(11) In the 11th aspect, in double-side image printing, offsetcorrection is executed by detecting offsets (error information) at threeor more positions in the main scanning direction using the distancesbetween test patterns in test image data or the distances between testpatterns printed in the other color as reference distances. This makesit possible to properly adjust a pixel offset (error information) in anintermediate portion as well as aligning the starts and ends of lines.In addition, a pixel offset between the respective colors on the obverseand reverse surfaces can also be properly adjusted.

(12) In the 12th aspect, since the frequency of a dot clock can bechanged even during scanning of one line under the control of the clockcontrol section, not only the starts and ends of lines can be aligned,but also a pixel offset (halfway offset) in an intermediate portion canbe adjusted. Therefore, an image can be printed without any halfwayoffset at a low cost.

(13) In the 13th aspect, since the frequency of a dot clock can bearbitrarily changed even during scanning of one line by selecting adesired one of delay clocks delayed the delay line, not only the startsand ends of lines can be aligned, but also a pixel offset (halfwayoffset) in an intermediate portion can be adjusted. Therefore, an imagecan be printed without any halfway offset at a low cost.

(14) In the 14th aspect, offset adjustment in the main scanningdirection can be properly performed by setting offsets so as to a lowthe frequency of a dot clock to be changed during scanning of one linein the main scanning direction.

(15) In the 15th aspect, in image printing, offset correction in themain scanning direction can be properly performed by changing thefrequency of a dot clock during scanning of one line in the mainscanning direction on the basis of at least two offsets set before imageprinting.

(16) In the 16th aspect, in image printing, offset correction in themain scanning direction can be properly performed by controlling thefrequency of a dot clock so as to make the distances between testpatterns at three positions coincide with each other.

(17) In the 17th aspect, in image printing, offset correction in themain scanning direction can be properly performed by controlling thefrequency of a dot clock during scanning of one line in the mainscanning direction so as to make the distances between test patterns atthree positions coincide with each other.

(18) In the 18th aspect, the frequency of a dot clock can be set todifferent frequencies depending on whether an image is to be printed onthe obverse or reverse surface of an image recording sheet. This makesit possible to adjust an offset between the two surfaces.

(19) In the 19th aspect, the frequency of a dot clock can be set todifferent frequencies depending on whether images are to be printed onthe two surfaces by repeatedly forming and fixing a toner image on eachof the surfaces or an image is to be printed on the reverse surface ofan image recording sheet. This makes it possible to properly adjust anoffset between the two surfaces.

(20) In the 20th aspect, the frequency of a dot clock can be set to agiven frequency for at least one surface depending on whether images areto be printed on the two surfaces by repeatedly forming and fixing atoner image on each of the surfaces or an image is to be printed on thereverse surface of an image recording sheet. This makes it possible torelatively make adjustment on the two surfaces and hence properly adjustan offset between the two surfaces.

The above and many other objects, features and advantages of the presentinvention will become manifest to those skilled in the art upon makingreference to the following detailed description and accompanyingdrawings in which preferred embodiments incorporating the principle ofthe invention are shown by way of illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bock diagram showing the electrical arrangement of the mainpart of an image printing apparatus according to the first embodiment ofthe present invention;

FIG. 2 is a flow chart for explaining the operation state of the imageprinting apparatus according to the first embodiment of the presentinvention;

FIG. 3 is a timing chart for explaining how offset detection isperformed in the image printing apparatus according to the firstembodiment of the present invention;

FIG. 4 is a timing chart for explaining the operation of the imageprinting apparatus according to the first embodiment of the presentinvention;

FIG. 5 is a timing chart for explaining the operation state of the imageprinting apparatus according to the first embodiment of the presentinvention;

FIG. 6 is a timing chart for explaining the operation state of the imageprinting apparatus according to the first embodiment of the presentinvention;

FIG. 7 is a sectional view showing the arrangement of a writing sectionof the image printing apparatus according to the first embodiment of thepresent invention;

FIG. 8 is a sectional view showing the arrangement of a writing sectionof an image printing apparatus according to the second embodiment of thepresent invention;

FIG. 9 is a flow chart for explaining the operation of the imageprinting apparatus according to the second embodiment of the presentinvention;

FIG. 10 is a sectional view showing the arrangement of a writing sectionof an image printing apparatus according to the third embodiment of thepresent invention;

FIG. 11 is a block diagram showing the electrical arrangement of themain part of the image printing apparatus according to the thirdembodiment of the present invention;

FIG. 12 is a flow chart for explaining the operation of the imageprinting apparatus according to the third embodiment;

FIGS. 13A to 13E are schematic views for explaining an example of dotclock adjustment in the present invention; and

FIG. 14 is a perspective view showing a general arrangement of a writingsection of an image printing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A few preferred embodiments of the present invention will be describedbelow with reference to the accompanying drawings.

Note that in the present invention, changing the frequency of a dotclock means changing the leading or trailing edge timing of the dotclock to a desired timing such as a given timing during scanning of oneline or a timing corresponding to each image printing surface or imageprinting color in order to change the position of a pixel printed inaccordance with the dot clock. As techniques of changing the frequencyof a dot clock, there are available a technique of changing or switchingthe frequency of the dot clock at an arbitrary timing and a technique ofgenerating a dot clock whose timing changes by selecting one of aplurality of dot clocks with the same frequency and different timings(phases).

The following embodiment will be described by exemplifying the use ofthe technique of generating dot clocks which change in timing byselecting dot clocks from a plurality of dot clocks with the samefrequency and different timings (phases).

First Embodiment

The overall arrangement of an image printing apparatus to which thefirst embodiment is applied will be described first.

The image printing apparatus scans a laser beam modulated in accordancewith image data in the main scanning direction, and forms an image on animage carrier which rotates in the sub-scanning direction. In this case,the laser beam is modulated with the image data with reference to apixel clock called a dot clock. FIG. 7 is a view showing the arrangementof the print engine of the image printing apparatus according to thisembodiment.

FIG. 7 shows the image printing apparatus capable of printing images onthe two surfaces of an image recording sheet as a specific example. Notethat the image printing apparatus capable of printing images on the twosurfaces of an image recording sheet can print an image on only onesurface of an image recording sheet.

Referring to FIG. 7, reference numeral 10 denotes a photosensitive drumserving as an image carrier on which a toner image is formed; and 11, acharging electrode for applying a predetermined potential to thephotosensitive drum 10.

Reference numeral 20 denotes a writing unit for forming an electrostaticlatent image on the surface of the photosensitive drum 10 by scanning alaser beam in accordance with image information.

Reference numeral 30 denotes a developing device for forming a tonerimage by developing the electrostatic latent image formed on the surfaceof the photosensitive drum 10 with a developer (toner).

Reference numeral 40 denotes a transfer/separation electrode fortransferring the toner image from the photosensitive drum 10 to an imagerecording sheet, and separating the transfer sheet from thephotosensitive drum 10. Note that an illustration of a cleaning sectionand the like is omitted.

Reference numeral 50 denotes a paper feed section in which imagerecording sheets are stored. FIG. 7 shows paper feed cassettes 51 and 52of the paper feed section 50. Note that the number of paper feedcassettes is not limited to this.

Reference numerals 61 to 68 denote convey devices such as convey rollersand path switching device. Reference numeral 61 denotes a feed rollerfor feeding an image recording sheet from a paper feed cassette; 62, aconvey path along which an image recording sheet from the paper feedsection 50 (51 or 52) travels; 63 a and 63 b, registration rollers forfeeding an image recording sheet at the timing of image printing; 65, apath switching device which switches between delivery and circulation ofan image recording sheet; 66 a, 66 b, 67 a, 67 b, 68 a, and 68 b,inversion rollers for inverting the obverse and reverse surfaces of animage recording sheet; and 69 a and 69 b, delivery rollers fordelivering an image recording sheet outside the apparatus.

Reference numeral 70 denotes a fixing unit for fixing (heat fusing) atoner image on an image recording sheet with heat and pressure. Thefixing unit 70 fixes a toner image on an image recording sheet whileclamping/conveying it with a heat roller 71 (or 72) and a pressureroller 72 (or 71).

The first embodiment of the image printing apparatus will be describedin detail below with reference to the accompanying drawings. Note thatthe image printing apparatus shown in FIGS. 14 and 7 is assumed as aspecific example of the image printing apparatus according to thisembodiment. That is, the image printing apparatus of the firstembodiment is an image printing apparatus capable of printing an imageon at least one surface of an image recording sheet. Note that thisapparatus may either be a monochrome image printing apparatus or colorimage printing apparatus.

The first embodiment of the image printing apparatus will be describedin detail below.

FIG. 1 shows a writing unit circuit section 200, an LD 260 for exposure,an index sensor 266, and a halfway offset sensor 270 a.

The writing unit circuit section 200 of the writing unit 20 forms anelectrostatic latent image on the surface of the photosensitive drum 10(see FIG. 7) by scanning a laser beam in accordance with image data.

Referring to FIG. 1, the writing unit circuit section 200 includes a CPU201 serving as a controller for controlling the overall image printingapparatus, a dot clock adjusting section 210 which is a characteristicfeature of the first embodiment, an image processing section 220 forperforming image processing, and an LD driving section 230 forgenerating an LD driving signal in accordance with a dot clock on thebasis of an image processing result.

Assume that the halfway offset sensor 270 a is placed downstream of thefixing unit 70 to detect pixel offsets at three or more positions in themain scanning direction and read predetermined patterns after fixing.

In this case, the predetermined pattern means a test image printed inaccordance with test image data stored in advance.

The arrangement and operation of the dot clock adjusting section 210will be sequentially described below. Not that the dot clock adjustingsection 210 is a clock generating section in the claims. The dot clockadjusting section 210 is comprised of a clock generating section 211 forgenerating a fundamental clock and a frequency changing section 212capable of changing the frequency of the fundamental clock generated bythe clock generating section 211. In this case, the frequency changingsection 212 changes the leading or trailing edge timing or frequency ofa dot clock to a desired timing such as a given timing during scanningof one line or a timing corresponding to each image printing surface orimage printing color in order to change the position of a pixel printedin accordance with the dot clock. In this embodiment, the frequencychanging section 212 is formed from a digital delay type dot clockadjusting device, and is comprised of (A), (B), (C), and (D) describedbelow.

(A) Delay Signal Generation:

A delay chain section 213 is a delay element group for obtaining aplurality of delay signals (delay signal group: in FIG. 1) havingslightly different phases from each other by delaying an input signal(the fundamental clock from the clock generating section 211).

In this case, in the delay chain section 213, delay elements arepreferably cascaded in the form of a chain to connect a sufficientnumber of delay elements to generate delay signals having slightlydifferent phases from each other over two fundamental clock periods.

Although the clock generating section 211 may be incorporated in each ofclock generating circuits for the respective colors, i.e., Y, M, C, andK, in a case of a color image printing apparatus, fundamental clocks maybe distributed from the single clock generating section 211 to the clockgenerating circuits for the respective colors.

As has already been shown in FIG. 14, the index sensor 266 detects anexposure timing at a reference position in scanning of a laser beam, andgenerates an index signal indicating the timing.

(B) Synchronization Detection:

A synchronization detecting section 214 is a detecting section whichreceives the index signal generated by the index sensor 266 and detectsthe ordinal number of a delay signal (synchronization point), of a delaysignal group (in FIG. 1), which is synchronized with the index signal.This section outputs synchronization point information (in FIG. 1).

In this case, the synchronization detecting section 214 preferablyoutputs first synchronization point information SP1 indicating a delaysignal, of the delay signal group (in FIG. 1), which is synchronizedfirst with the index signal, and second synchronization pointinformation SP2 indicating a delay signal synchronized next with theindex signal.

Since a plurality of delay signals from the delay chain section 213 maygreatly vary in delay time due to the influences of temperature changesand the like, how many delay signals are included in a predeterminedunchanging time (the time interval between an index signal and anotherindex signal) is detected in advance in this manner. This makes itpossible to calculate back the delay time per delay signal of a delaysignal group.

(C) Selection Control:

A selection control section 215 obtains a synchronization correctionamount on the basis of synchronization point information (in FIG. 1)from the synchronization detecting section 214 and frequency offsetinformation (in FIG. 1) from the CPU 201, and outputs a select signal(in FIG. 1) indicating a delay signal having a specific phase which isto be selected from a delay signal group (in FIG. 1). The frequencyoffset information will be described later.

(D) Selection:

A selecting section 216 receives a select signal in FIG. 1) from theselection control section 215, and selects a delay signal having acorresponding phase from a delay signal group (in FIG. 1). The selectingsection 216 then generates a rectangular wave by setting the selecteddelay signal as leading and trailing edges, and outputs the rectangularwave as a dot clock (in FIG. 1).

In this manner, the period of a dot clock is slightly increased ordecreased by setting a delay signal having a selected timing as leadingand trailing edges in generating a dot clock. This can realize (a) and(b) described below.

(a) Generating a signal having a predetermined number of pulsesgenerated in a predetermined period of time makes the length of eachline in the main scanning direction uniform and makes the length betweenthe start and the end of each line constant.

(b) The timing of a dot clock is adjusted at three or more positions inthe main scanning direction by using pieces of error information at thethree or more positions in the main scanning direction, therebysuppressing a halfway offset.

Not only (a) but also (b), which is unfeasible in the prior art, can berealized by sequentially selecting delay signals, whose phases (thepositions or timings of the pulses of a dot clock) are finely andgradually changed, within a predetermined period of time withoutchanging the clock frequency itself instead of making adjustment byfinely adjusting the oscillation frequency of a fundamental clockoscillated and generated by the clock generating section 211.

<Principle (1) of Offset Detection for Dot Clock Adjustment>

The manner of offset detection in the first embodiment will be brieflydescribed below with reference to the flow chart of FIG. 2 and theschematic view of FIG. 3.

At the time of offset detection, the writing unit circuit section 200generates images with predetermined patterns stored in a ROM 221 at atotal of three or more positions, i.e., the start position, endposition, and intermediate position on the photosensitive drum 10 in themain scanning direction (S1 in FIG. 2). FIG. 3 shows a case whereinpatterns in the form of the Katakana character “

” are printed as predetermined patterns at three positions in the mainscanning direction.

Assume that although the patterns indicated by the solid lines areactually printed on the image recording sheet, the patterns indicated bythe broken lines were expected to be printed. In this case, offsets dx1,dx2, and dx3 have occurred with respect to the respective patterns inthe main scanning direction due to the aberrations of the writing unitand the respective optical systems, contraction due to fixing, and thelike.

When these patterns are read by the halfway offset sensor 270 a disposedat a position where it can read the patterns on the image recordingsheet having passed through the fixing unit 70 (S2 in FIG. 2), adistance Y1 from the horizontal line to the oblique line of the patternin the form of the Katakana character “

” includes an offset dy1, and distances Y2′ and Y3′ respectively includeoffsets dy2 and dy3.

Letting è be the angle defined by the horizontal and oblique lines,dx1=dy1/tan è. In addition, the offset dy1 can also be obtained from themoving speed of the image carrier in the sub-scanning direction and thedifference between the times when the horizontal and oblique lines areread.

Likewise, letting è be the angle defined by the horizontal and obliquelines, dx2=dy2/tan è. Furthermore, the offset dy2 can be obtained fromthe moving speed of the image carrier in the sub-scanning direction andthe difference between times when the horizontal and oblique line areread.

Likewise, letting è be the angle defined by the horizontal and obliquelines, dx3=dy3/tan è. Furthermore, the offset dy3 can be obtained fromthe moving speed of the image carrier in the sub-scanning direction andthe difference between times when the horizontal and oblique line areread.

In the above manner, the positions (offset states) of the test patternsare calculated by the CPU 201 (S3 in FIG. 2).

The CPU 201 compares the positions (offset states) of the test patternswith the reference distances based on the test patterns to calculatepixel offset states within the image recording sheet surface (S4 in FIG.2).

Therefore, by printing and reading such predetermined patterns on onesurface of an image recording sheet at the same position in thesub-scanning direction and three or more positions in the main scanningdirection, the CPU 201 can detect offset states (frequency offsetinformation) associated with the expansion/contraction of an image andpixel offsets in the main scanning direction.

The CPU 201 performs the above detection processing with respect to onesurface of the image recording sheet in this manner, and suppliesfrequency offset information (in FIG. 1) to the selection controlsection 215 of the writing unit circuit section 200.

Likewise, the CPU 201 can obtain image leading end offset informationassociated with the start position of an image in the main scanningdirection by detecting the pattern in the form of the Katakana character“

” on the start end side in the main scanning direction, and can supplythe image leading end offset information to the selection controlsection 215 in the writing unit circuit section 200.

In the above manner, in usual image printing operation, on the basis offrequency offset information representing a pixel offset state on onesurface, the dot clock adjusting section 210 generates a dot clock foreliminating a halfway offset which is the pixel offset state on onesurface of the image recording sheet, thereby printing an image (S5 inFIG. 2)

<Operation of Image Printing Apparatus>

The operation of the image printing apparatus according to the firstembodiment will be described next with reference to the timing chart ofFIG. 4.

<Operation of Digital Delay Type Dot Clock Adjustment>

The operation of making adjustment to set the pulse count to apredetermined count by shifting the pulse of a fundamental clock withreference to offset information and to set the time during which thepredetermined number of pulses are generated to a predetermined time,and suppressing a halfway offset by adjusting the dot clock timing atthree or more positions in the main scanning direction will be describedup to the step of generating a dot clock, first, with reference to FIG.4.

The CPU 201 supplies a correction amount computing section in theselection control section 215 with offset information representing anoffset ER detected by the above printing and reading of thepredetermined patterns, clock frequency information of a clock period TCobtained from the frequency of a fundamental clock, one-line pixel countinformation representing a pixel count PH of pixels to be printed in themain scanning direction.

In addition, a synchronization count (a count by which a delay of onefundamental clock period) NS is obtained from the first synchronizationpoint information SP1 and second synchronization point information SP2from the synchronization detecting section 214.

In this case, the correction amount computing section in the selectioncontrol section 215 obtains a correction count value (count load data)CC corresponding to a correction amount by

CC=PH×(NS/TC)/ER

This correction count value CC is counted down by a switching countdevice in the selection control section 215 to switch between a selectsignal and a low-order select signal. As the correction amountincreases, therefore, the correction count value CC decreases.

The synchronization detecting section 214 refers to the leading edge ofan index signal from the index sensor 266 to obtain, as synchronizationpoint information, the ordinal number of a delay element of the delaychain section 213 at which a delay signal synchronized with the leadingedge of the index signal is obtained.

Assume that 20 and 50 are obtained as the first synchronization pointinformation SP1 and second synchronization point information SP2,respectively. In this case, the above synchronization count NS becomes30.

In this case, an index signal is generated at the timing when the indexsensor detects a laser beam scanned by the writing unit (T1 of (a) inFIG. 4). Thereafter H_VALID representing a valid area in the horizontaldirection is activated.

The switching count device in the selection control section 215 keepscounting down the correction count value CC in accordance with afundamental clock. Every time the count values is counted down to 0,count data is supplied as an interrupt to a select signal computingsection 443 in the selection control section 215 ((d) to (f) in FIG. 4).

The CPU 201 supplies offset direction information to the select signalcomputing section in the selection control section 215. Morespecifically, the CPU 201 supplies “−correction” information for makingcorrection to contract an offset as image expansion in the main scanningdirection, and “+correction” information for making correction to expandan offset as image contraction.

Assume that offset information ER and offset direction information havebeen obtained by printing and measuring the predetermined patternsdescribed above. In this case, ER=6 ns and offset directioninformation=“−correction”. That is, assume that the obtained informationindicates making correction to contract the image because of itsexpansion.

First of all, the synchronization detecting section 214 obtains thefirst synchronization point information SP1 and second synchronizationpoint information SP2 by referring to the leading edge of an indexsignal from the index sensor 266.

The first synchronization point information SP1 indicates the ordinalnumber of a delay element of the delay chain section 213 which issynchronized with the leading edge of the index signal. The secondsynchronization point information SP2 indicates the ordinal number of adelay element of the delay chain section 213 which lags behind the firstsynchronization point information SP1 by one fundamental clock period.

Assume that SP1=20 and SP2=50. FIG. 5 shows this state. In this case,20th DL 20 ((c) in FIG. 5) and 50th DL 50 ((m) in FIG. 5) lagging behindDL 20 by one clock period are synchronized with the leading edge of theindex signal ((a) in FIG. 5).

The synchronization count NS is then obtained from the firstsynchronization point information SP1 and second synchronization pointinformation SP2. In this case, the synchronization count NS indicates aspecific number of delay elements to which a time of one fundamentalclock period corresponds in terms of delay time. In the firstembodiment, NS=30 according to synchronization count NS=SP2−SP1.

A delay time DT of per delay element is obtained from NS and thefundamental clock period. Assume that a fundamental clock period TC is30 ns. In this case, since NS=30, DT=1 ns according to DT=TC/NS. Thedelay time per delay element varies due to variations in the temperatureof an integrated circuit, the power supply voltage applied to theintegrated circuit, and the like, and hence may become 1.5 ns in somecase and 0.5 ns in another case. However, since the fundamental clockperiod TC is constant, the delay time per delay element at the time ofmeasurement can be accurately obtained by obtaining the synchronizationcount NS.

The correction count value CC indicating how many delay elements bywhich a shift should be finally made to obtain a proper image signal isobtained from the offset information ER, offset direction information,and delay time DT. In this case, ER=6 ns, offset directioninformation=“−correction”, and DT=1 ns. Therefore, correction countvalue CC=−6.

According to the above correction count value CC, in order to obtain aproper image signal, the delay element count may be finally advanced bysix. That is, a signal from the 50th delay element is used insynchronism with the leading edge of an index signal, and signals fromthe 49th, 48th, 47th, 46th, and 45th delay elements are sequentiallyinterchanged and used in synchronism with a select signal. Finally, asignal from the 44th delay element is used.

If the correction amount is larger than the synchronization count, aselect signal may be circulated. In a case of “− correction” withSP1=20, SP2=50, and synchronization count=30, when a select signal isset to 50, 49, . . . , 21, and 20, since 20 of the select signal is inphase with 50 of the select signal, the select signal is set next to 49,48, . . . . That is, the select signal is set to 50, 49, . . . , 21, 20(=50), 49, 48, . . . . In a case of “+correction” as well, a selectsignal may be circulated in the same manner.

When “− correction” is to be made in steps of three like 50, 47, 43, . .. , 23, 19, SP1=20 will be exceeded. However, 50−(20−19)−3=46 is setnext to 19. That is, the select signal can be circulated without anyproblem while an excess over the synchronization point and onecorrection amount are added together.

Upon reception of such a select signal, the selecting section 216selects the 50th signal, 49th signal, 48th signal, 47th signal, . . .from a delay signal group (in FIG. 1) from the delay chain section 213,and outputs them as dot clocks ((g) in FIG. 4).

When the 50th signal, 49th signal, 48th signal, 47th signal, . . . areselected from the delay signal group (in FIG. 1), a delay signalsynchronized with an index signal is obtained first. Subsequently, delaysignals which gradually decrease in delay (advancing in phase) areobtained. As a result, “− correction” is realized to make correction tocontract an offset as image expansion in the main scanning direction.

In a case of “+ correction”, by selecting the 20th signal, 21st signal,22nd signal, 23rd signal, . . . from the delay signal group (in FIG. 1),with the first synchronization point information SP1 being set as aninitial value, a delay signal synchronized with an index signal isobtained first. Subsequently, delay signals which gradually decrease indelay (lagging in phase) are obtained. As a result, “+ correction” isrealized to make correction to expand an offset as image contraction inthe main scanning direction.

That is, adjustment can be made to set the pulse count to apredetermined count by shifting the pulse of a fundamental clock atgiven time intervals and to set the time during which the predeterminednumber of pulses are generated to a predetermined time.

Since the above correction is controlled on the basis of the offsetinformation ER (frequency offset information), an accurate adjustment ismade for a length in the main scanning direction, and a halfway offsetcan be suppressed.

FIG. 6 schematically shows how the above correction ofexpansion/contraction in the main scanning direction (i.e.,main-scanning magnification correction) is made. FIG. 6 shows afundamental clock, delay signals (first to ninth delay clocks), and adot clock.

In the case shown in FIG. 6, by selecting the first delay clock, seconddelay clock, third delay clock, fourth delay clock, fifth delay clock, .. . during four fundamental clock periods, 3.5 dot clocks are set duringfour periods. That is, 3.5/4=87.5%, and hence control is performed todecrease the frequency in a pseudo manner. Note that the same result canbe obtained by executing another selection method.

In the case shown in FIG. 6, since the eighth delay clock is in phasewith the fundamental clock, when the eighth delay clock, seventh delayclock, sixth delay clock, fifth delay clock, fourth delay clock, . . .are selected during four fundamental clock periods, 4.5 dot clocks (notshown) are set during four periods. That is, 4.5/4=112.5%, and hencecontrol is performed to increase the frequency in a pseudo manner. Notethat the same result can be obtained by executing another selectionmethod.

In this case, a specific manner of adjusting a dot clock in thisembodiment will be schematically shown and described. FIGS. 13A to 13Eshow a case wherein test patterns are printed at four differentpositions in the main scanning direction (to in FIG. 13A).

Assume that a halfway offset is detected by the halfway offset sensor270 a by the test patterns in this state, as shown in FIG. 13B.

Assume that no halfway offset has occurred at the position of in FIG.13A, a halfway offset has occurred at the position of (2) in FIG. 13A(in a direction toward the left side of the drawing surface and in whichthe interval between (1) and (2) decreases in FIG. 13A), no halfwayoffset has occurred at the position (3) in FIG. 13A, and no halfwayoffset has occurred at the position (4) in FIG. 13A.

In this case, the interval between (1) and (2) has decreased. Since nohalfway offset has occurred at (3) the interval between (2) and (3) hasincreased. In addition, since no halfway offset has occurred at (4), thelength in the main scanning direction is assumed to be a predeterminedlength.

When the above halfway offset has occurred, the CPU 201 performs controlto increase the interval between dot clocks between (1) and (2) by theabove “+ correction”. Likewise, the CPU 201 performs control to decreasethe interval between dot clocks between (2) and (3) by the above “−correction”. Since no change in interval has occurred between (3) and(4), no correction is performed. With the above operation, the halfwayoffset can be eliminated, and a state wherein there is no change inlength in the main scanning direction can be obtained.

As shown in FIG. 13C, “+ correction” and “− correction” may be performedstepwise. In addition, as shown in FIG. 13D, “+ correction” and “−correction” are preferably performed more finely and smoothly ifpossible in terms of correction step.

Note that FIGS. 13A to 13E show an example of a halfway offset, whichcan be eliminated to make the main-scanning length constant by applyingthis embodiment and performing proper correction in accordance with thehalfway offset.

The use of the digital delay type dot clock adjusting section in theabove manner can adjust an offset between the start and the end in themain scanning direction. A halfway offset can be suppressed by adjustingthe timing of a dot clock at three or more positions in the mainscanning direction using pieces of error information at three or morepositions on one surface of an image recording sheet in the mainscanning direction. In the field of offset printing or the like, inparticular, not only expansion/contraction between the start and the endbut also a pixel offset caused at a halfway point (halfway offset) maypose a serious problem. However, satisfactory offset correction can beperformed by this embodiment of the present invention.

The above specific example has exemplified the case of the test patternsat the three positions. However, increasing the number of test patternswill suppress halfway offsets more finely. If, for example, testpatterns are printed at four positions, halfway offset suppression canbe done in areas on the left end portion/near the center/on the rightend portion. That is, offset correction can be done at three positionsby using test patterns at four positions, thereby obtaining a betterresult.

The first embodiment uses the digital circuit arrangement having noanalog feedback circuit such as a PLL. This circuit and other digitalcircuit portions can therefore be integrated into one chip. This makesit possible to perform halfway offset elimination with higher precision.

Letting L be the number of pixels per line in the main scanningdirection, which are used for image printing L1 to Ln−1 be the numbersof pixels in the respective ranges divided by n test patterns (n isequal to or more than 3), and N1 to Nn−1 be the offset amountscalculated between the three or more test patterns, it is preferablethat an offset correction amount Nj/Lj (where 1 j n) between each pairof test patterns be evenly distributed to the respective pixels betweeneach pair of test patterns so as to be reflected in the selection of adelay count for a dot clock.

In this manner, error information is detected at three or more positionsin the main scanning direction and is evenly distributed to therespective pixels between each pair of test patterns so as to bereflected in the selection of a delay count for a dot clock. This canalign the starts and the ends of the respective lines and properlysuppress a halfway offset as a pixel offset in a halfway portion.

In the first embodiment, the CPU 201 is externally provided for the dotclock adjusting section 210. However, the dot clock adjusting section210 may incorporate controllers such as the CPU 201 and various kinds oftables.

As described above in detail, according to the first embodiment, when adot clock as the basis of each pixel forming an image is generated andexposure of each pixel is to be performed on one surface of an imagerecording sheet on the basis of the dot clock, a plurality of delayclocks are generated by finely delaying a fundamental clock, and themanner of selection for the plurality of delay clocks is changed tochange the leading edge timing or trailing edge timing of the dot clockgenerated for a surface on which an offset should be adjusted. In thisstate, test patterns are printed at three or more positions on an imageprinting surface in the main scanning direction to detect errorinformation associated with an offset on one surface, and the manner ofselection of a plurality of delay clocks is controlled in accordancewith the error information, thereby correcting an offset on one surface.

Since this embodiment uses the digital circuit arrangement having noanalog feedback circuit such as a PLL, the circuit and other digitalcircuit portions can be integrated into one chip. This makes it possibleto perform halfway offset elimination with higher precision.

In addition, in the first embodiment, offset correction is executed upondetection of error information at three or more positions in the mainscanning direction. This can align the starts and the ends of therespective lines in the main scanning direction and adjust a halfwayoffset as a pixel offset in a halfway portion between the start and theend of each line.

In the first embodiment, the offset detecting section, digital delaytype dot clock adjusting device, and controller can be formed fromdigital circuits. These sections can perform halfway offset eliminationwith high precision in a state wherein they are suitable for integrationinto one chip.

The first embodiment described above is directed to halfway offsetcorrection. If, however, a pattern in the form of the Katakana character“

” to be printed on the start side in the main scanning direction isplaced as near the end portion at the start position in the mainscanning direction as possible, and image leading end offset informationassociated with the start position of an image in the main scanningdirection is supplied to the selection control section 215 in thewriting unit circuit section 200, the start positions of the respectivelines in the main scanning direction can be aligned.

According to the above description of the first embodiment, a halfwayoffset is detected by the halfway offset sensor 270 a, and an offset isset. In addition to this operation, the CPU 201 may analyze the imagedata obtained by making a scanner (not shown) read an image recordingsheet on which test patterns are printed, and set an offset.Alternatively, an operator may measure an image recording sheet on whichtest patterns are printed, and set an offset obtained from themeasurement result with respect to the CPU 201 with an operating section(not shown).

Second Embodiment

The second embodiment of the present invention will be described indetail below with reference to the accompanying drawings. Note that theimage printing apparatus shown in FIGS. 14 and 7 is assumed to be aspecific example of an image printing apparatus according to the secondembodiment. That is, the image printing apparatus according to thesecond embodiment is an image printing apparatus capable of printingimages on the two surfaces of an image recording sheet. Note that thisapparatus may either be a monochrome image printing apparatus or colorimage printing apparatus.

The image printing apparatus according to the second embodiment of thepresent invention will be described in detail below.

FIG. 8 shows a writing unit circuit section 200, an LD 260 forperforming exposure, an index sensor 266, and an obverse/reverse pixeloffset sensor 270 b.

The writing unit circuit section 200 of a writing unit 20 forms anelectrostatic latent image by scanning a laser beam on the surface of aphotosensitive drum 10 (see FIG. 7) in accordance with image data.

Referring to FIG. 8, the writing unit circuit section 200 includes a CPU201 serving as a controller for controlling the overall image printingapparatus, a dot clock adjusting section 210 which is a characteristicfeature of the second embodiment, an image processing section 220 forperforming image processing, and an LD driving section 230 forgenerating an LD driving signal in accordance with a dot clock based onthe image processing result.

Assume that the obverse/reverse pixel offset sensor 270 b is placeddownstream of a fixing unit 70 to detect pixel offsets at three or morepositions in the main scanning direction and read predetermined patternsafter fixing.

In this case, the predetermined pattern means a test image printed inaccordance with test image data stored in advance.

The arrangement and operation of the dot clock adjusting section 210will be sequentially described below. Not that the dot clock adjustingsection 210 is a clock generating section in the claims. The dot clockadjusting section 210 is comprised of a clock generating section 211 forgenerating a fundamental clock and a frequency changing section 212capable of changing the frequency of the fundamental clock generated bythe clock generating section 211. In this case, the frequency changingsection 212 changes the leading or trailing edge timing or frequency ofa dot clock to a desired timing such as a given timing during scanningof one line or a timing corresponding to each image printing surface orimage printing color n order to change the position of a pixel printedin accordance with the dot clock. In this embodiment, the frequencychanging section 212 is formed from a digital delay type dot clockadjusting device, and is comprised of (A), (B), (C), and (D) describedbelow.

(A) Delay Signal Generation:

A delay chain section 213 is a delay element group for obtaining aplurality of delay signals (delay signal group: in FIG. 8) havingslightly different phases from each other by delaying an input signal(the fundamental clock from the clock generating section 211).

In this case, in the delay chain section 213, delay elements arepreferably cascaded in the form of a chain to connect a sufficientnumber of delay elements to generate delay signals having slightlydifferent phases from each other over two fundamental clock periods.

Although the clock generating section 211 may be incorporated in each ofclock generating circuits for the respective colors, i.e., Y, M, C, andK in a case of a color image printing apparatus, fundamental clocks maybe distributed from the single clock generating section 211 to the clockgenerating circuits for the respective colors.

As has already been shown in FIG. 14, the index sensor 266 detects anexposure timing at a reference position in scanning of a laser beam, andgenerates an index signal indicating the timing.

(B) Synchronization Detection:

A synchronization detecting section 214 is a detecting section whichreceives the index signal generated by the index sensor 266 and detectsthe ordinal number of a delay signal (synchronization point), of a delaysignal group (in FIG. 8), which is synchronized with the index signal.This section outputs synchronization point information (in FIG. 8).

In this case, the synchronization detecting section 214 preferablyoutputs first synchronization point information SP1 indicating a delaysignal, of the delay signal group (in FIG. 8), which is synchronizedfirst with the index signal, and second synchronization pointinformation SP2 indicating a delay signal synchronized next with theindex signal.

Since a plurality of delay signals from the delay chain section 213 maygreatly vary in delay time due to the influences of temperature changesand the like, how many delay signals are included in a predeterminedunchanging time (the time interval between an index signal and anotherindex signal) is detected in advance in this manner. This makes itpossible to calculate back the delay time per delay signal of a delaysignal group.

(C) Selection Control:

A selection control section 215 obtains a synchronization correctionamount on the basis of synchronization point information (in FIG. 8)from the synchronization detecting section 214 and frequency offsetinformation (in FIG. 8) from the CPU 201, and outputs a select signal(in FIG. 8) indicating a delay signal having a specific phase which isto be selected from a delay signal group (in FIG. 8). The frequencyoffset information will be described later.

(D) Selection:

A selecting section 216 receives a select signal in FIG. 8) from theselection control section 215, and selects a delay signal having acorresponding phase from a delay signal group (in FIG. 8). The selectingsection 216 then generates a rectangular wave by setting the selecteddelay signal as leading and trailing edges, and outputs the rectangularwave as a dot clock (in FIG. 8).

In this manner, the period of a dot clock is slightly increased ordecreased by setting a delay signal having a selected timing as leadingand trailing edges in generating a dot clock. This can realize (a) and(b) described below.

(a) Generating a signal having a predetermined number of pulsesgenerated in a predetermined period of time makes the length of eachline in the main scanning direction uniform and makes the length betweenthe start and the end of each line constant.

(b) The timing of a dot clock is adjusted at three or more positions inthe main scanning direction by using pieces of error information at thethree or more positions in the main scanning direction, therebysuppressing a halfway offset.

Not only (a) but also (b), which is unfeasible in the prior art, can berealized by sequentially selecting delay signals, whose phases (thepositions or timings of the pulses of a dot clock) are finely andgradually changed, within a predetermined period of time withoutchanging the clock frequency itself instead of making adjustment byfinely adjusting the oscillation frequency of a fundamental clockoscillated and generated by the clock generating section 211.

<Principle (2) of Offset Detection for Dot Clock Adjustment>

The manner of offset detection in the first embodiment will be brieflydescribed below with reference to the flow chart of FIG. 9 and theschematic view of FIG. 3. At the time of offset detection, the writingunit circuit section 200 generates images with predetermined patternsstored in a ROM 221 at a total of three or more positions, i.e., thestart position, end position, and intermediate position on thephotosensitive drum 10 in the main scanning direction (S1 in FIG. 9).FIG. 3 shows a case wherein patterns in the form of the Katakanacharacter “

” are printed as predetermined patterns at three positions in the mainscanning direction.

Assume that although the patterns indicated by the solid lines areactually printed on the image recording sheet, the patterns indicated bythe broken lines were expected to be printed. In this case, offsets dx1,dx2, and dx3 have occurred with respect to the respective patterns inthe main scanning direction due to the aberrations of the writing unitand the respective optical systems, contraction due to fixing, and thelike.

When these patterns are read by the obverse/reverse pixel offset sensor270 b disposed at a position where it can read the patterns on the imagerecording sheet having passed through the fixing unit 70 (S2 in FIG. 9),a distance Y1′ from the horizontal line to the oblique line of thepattern in the form of the Katakana character “

” includes an offset dy1, and distances Y2′ and Y3′ respectively includeoffsets dy2 and dy3.

Letting è be the angle defined by the horizontal and oblique lines,dx1=dy1/tan è. In addition, the offset dy1 can also be obtained from themoving speed of the image carrier in the sub-scanning direction and thedifference between the times when the horizontal and oblique lines areread.

Likewise, letting è be the angle defined by the horizontal and obliquelines, dx2=dy2/tan è. Furthermore, the offset dy2 can be obtained fromthe moving speed of the image carrier in the sub-scanning direction andthe difference between times when the horizontal and oblique line areread.

Likewise, letting è be the angle defined by the horizontal and obliquelines, dx3=dy3/tan è. Furthermore, the offset dy3 can be obtained fromthe moving speed of the image carrier in the sub-scanning direction andthe difference between times when the horizontal and oblique line areread.

In the above manner, the positions of the test patterns (offset states)are calculated by the CPU 201 (53 in FIG. 9).

The image recording sheet whose test patterns printed on one surface areread is caused to pass along a reversal convey path to also print testpatterns on the other surface in the same manner (S4 in FIG. 9). Theobverse/reverse pixel offset sensor 270 b reads the test patterns (S5 inFIG. 9). The positions of the test patters (offset states) are thencalculated (S6 in FIG. 9).

The CPU 201 calculates pixel offset states on the two surfaces bycomparing the positions (offset states) of the test patterns on therespective surfaces (S7 in FIG. 9).

Note that the CPU 201 may calculate pixel offset states on therespective surfaces by comparing the positions (offset states) of thetest patterns on the respective surfaces with the reference distancesbased on the test patterns.

Therefore, by printing and reading such predetermined patterns on thetwo surfaces of an image recording sheet at the same position in thesub-scanning direction and three or more positions in the main scanningdirection, the CPU 201 can detect offset states (frequency offsetinformation) associated with the expansion/contraction of an image andpixel offsets in the main scanning direction.

The CPU 201 performs the above detection processing with respect to thetwo surfaces of the image recording sheet in this manner, and suppliesfrequency offset information (in FIG. 8) to the selection controlsection 215 of the writing unit circuit section 200.

Likewise, the CPU 201 can obtain image leading end offset informationassociated with the start position of an image in the main scanningdirection by detecting the pattern in the form of the Katakana character“

” on the start end side in the main scanning direction, and can supplythe image leading end offset information to the selection controlsection 215 in the writing unit circuit section 200.

In the above manner, in usual image printing operation, on the basis offrequency offset information representing pixel offset states on the twosurfaces, the dot clock adjusting section 210 generates dot clocks foreliminating the pixel offset states on the two surfaces of the imagerecording sheet, thereby printing an image (S8 in FIG. 9)

By executing the above correction in the same manner as in the firstembodiment which has been described with reference to FIGS. 4 to 6,control is done on the basis of offset information ER (frequency offsetinformation), the length in the main scanning direction is accuratelyadjusted, and an obverse/reverse pixel offset is suppressed.

In this case, proper correction can be made for an obverse/reverse pixeloffset and the length in the main scanning direction by performingcontrol based on the characteristics shown in FIGS. 13A to 13E inaccordance with the obverse/reverse pixel offset that has currentlyoccurred.

The use of the digital delay type dot clock adjusting section in theabove manner can adjust an offset between the start and the end in themain scanning direction. An obverse/reverse pixel offset can besuppressed by adjusting the timing of a dot clock at three or morepositions in the main scanning direction using pieces of errorinformation at three or more positions on each of the obverse andreverse surfaces of an image recording sheet in the main scanningdirection.

The above specific example has exemplified the case of the test patternsat the three positions. However, increasing the number of test patternswill suppress obverse/reverse pixel offsets more finely. If, forexample, test patterns are printed at four positions, obverse/reversepixel offset suppression can be done in areas on the left endportion/near the center/on the right end portion. That is, offsetcorrection can be done at three positions by using test patterns at fourpositions, thereby obtaining a better result.

The second embodiment uses the digital circuit arrangement having noanalog feedback circuit such as a PLL. This circuit and other digitalcircuit portions can therefore be integrated into one chip. This makesit possible to perform obverse/reverse pixel offset elimination withhigher precision.

Letting L be the number of pixels per line in the main scanningdirection which are used for image printing, L1 to Ln−1 be the numbersof pixels in the respective ranges divided by n test patterns (n isequal to or more than 3), and N1 to Nn−1 be the offset amountscalculated between the three or more test patterns, it is preferablethat an offset correction amount Nj/Lj (where 1 j n) between each pairof test patterns be evenly distributed to the respective pixels betweeneach pair of test patterns so as to be reflected in the selection of adelay count for a dot clock.

In this manner, error information is detected at three or more positionsin the main scanning direction and is evenly distributed to therespective pixels between each pair of test patterns so as to bereflected in the selection of a delay count for a dot clock. This canalign the starts and the ends of the respective lines and properlyperform pixel offset adjustment in a halfway portion.

In the second embodiment described above, the CPU 201 is externallyprovided for the dot clock adjusting section 210. However, the dot clockadjusting section 210 may incorporate controllers such as the CPU 201and various kinds of tables.

As described above in detail above, according to the second embodiment,when a dot clock as the basis of each pixel forming an image isgenerated and exposure of each pixel is to be performed on the twosurfaces of an image recording sheet on the basis of the dot clock, aplurality of delay clocks are generated by finely delaying a fundamentalclock, and the manner of selection for the plurality of delay clocks ischanged to change the leading edge timing or trailing edge timing of thedot clock generated for a surface on which an offset should be adjusted.In this state, test patterns are printed at three or more positions oneach surface in the main scanning direction to detect error informationassociated with an offset on each surface, and the manner of selectionof a plurality of delay clocks is controlled in accordance with theerror information, thereby correcting an offset on one surface.

Since this embodiment uses the digital circuit arrangement having noanalog feedback circuit such as a PLL, the circuit and other digitalcircuit portions can be integrated into one chip. This makes it possibleto perform halfway offset elimination with higher precision.

In addition, in the second embodiment, offset correction is executedupon detection of error information at three or more positions in themain scanning direction. This can align the starts and the ends of therespective lines in the main scanning direction and adjust anobverse/reverse pixel offset as a pixel offset in a halfway portionbetween the start and the end of each line.

The second embodiment has the digital delay type dot clock adjustingdevice as a circuit common to the respective surfaces on which offsetsshould be adjusted. This makes it possible to reduce the circuit sizewhile eliminating an obverse/reverse pixel offset with high precision.

In the second embodiment, the digital delay type dot clock adjustingdevice and controller can be formed from digital circuits. Thesesections can perform obverse/reverse pixel offset elimination with highprecision in a state wherein they are suitable for integration into onechip.

In the second embodiment, since the apparatus operates under the controlof a controller such as a CPU externally provided for the digital delaytype dot clock adjusting device, an obverse/reverse pixel offset can beeliminated with high precision.

In the second embodiment, since the apparatus operates under the controlof a controller such as a CPU incorporated in the digital delay type dotclock adjusting device, obverse/reverse pixel offset elimination can bedone with high precision in a state wherein the device is suitable forintegration into one chip.

The second embodiment described above is directed to obverse/reversepixel offset correction. If, however, a pattern in the form of theKatakana character “

” to be printed on the start side in the main scanning direction isplaced as near the end portion at the start position in the mainscanning direction as possible, and image leading end offset informationassociated with the start position of an image in the main scanningdirection is supplied to the selection control section 215 in thewriting unit circuit section 200, the start positions of the respectivelines in the main scanning direction can be aligned.

According to the above description of the second embodiment, anobverse/reverse pixel offset is detected by the obverse/reverse pixeloffset sensor 270 b, and an offset is set. In addition to thisoperation, the CPU 201 may analyze the image data obtained by making ascanner (not shown) read an image recording sheet on which test patternsare printed, and set an offset. Alternatively, an operator may measurean image recording sheet on which test patterns are printed, and set anoffset obtained from the measurement result with respect to the CPU 201with an operating section (not shown).

Third Embodiment

The third embodiment of the present invention will be described below.The third embodiment is directed to a color image printing apparatushaving an exposure unit (see FIG. 14) for each of a plurality of colorsto print an image in a plurality of colors.

This color image printing apparatus scans a laser beam modulated inaccordance with image data in the main scanning direction for eachcolor, and forms an image in each color on an image carrier for eachcolor which rotates in the sub-scanning direction. In this case, thelaser beam is modulated with the image data with reference to a pixelclock called a dot clock.

FIG. 10 is a sectional view showing the arrangement of the print engineof the image printing apparatus according to the third embodiment.

FIG. 10 shows a specific example of the image printing apparatus whichcan print a color image by using toners of a plurality of colors. Inthis apparatus, toner image from the image carriers of the respectivecolors are primarily transferred to the intermediate transfer member andsuperimposed thereon. An image recording sheet is then clamped betweenthe intermediate transfer member and the transfer roller to secondarilytransfer the image from the intermediate transfer member to the imagerecording sheets thereby printing the image.

Reference numeral 10Y to 10K denote photosensitive drums serving asimage carriers on which Y (Yellow), M (Magenta), C (Cyan), and K (blacK)toner images are formed.

Reference numerals 20Y to 20K denote writing units for formingelectrostatic latent images on the surfaces of the photosensitive drums10Y to 10K by scanning laser beams in accordance with pieces of imageinformation of the respective colors Reference numerals 30Y to 30Kdenote developing devices for forming toner images by developing theelectrostatic latent images formed on the surfaces of the photosensitivedrums 10Y to 10K with developers (toners) o the respective colors.

Reference numeral 40 denotes an intermediate transfer member belt onwhich the toner images from the photosensitive drums 10Y to 10K of therespective colors are transferred (primarily transferred) andsuperimposed; and 64, a secondary transfer roller for transferring(secondarily transferring) the toner image from the Intermediatetransfer member belt 40 onto an image recording sheet, and separatingthe image recording sheet from the intermediate transfer member belt 40.Note that an illustration of a belt cleaning section and the like isomitted.

Reference numeral 50 denotes a paper feed section in which imagerecording sheets are stored. FIG. 10 shows paper feed cassettes 51 and52 of the paper feed section 50. Note that the number of paper feedcassettes is not limited to this.

Reference numerals 61 to 68 denote convey devices such as convey rollersand path switching device. Reference numeral 61 denotes a feed rollerfor feeding an image recording sheet from a paper feed cassette; 62, aconvey path along which an image recording sheet from the paper feedsection 50 (51 or 52) travels; 63 a and 63 b, registration rollers forfeeding an image recording sheet at the timing of image printing; 64, asecondary transfer roller; 65, a path switching device which switchesbetween delivery and circulation of an image recording sheet; 66 a, 66b, 67 a, 67 b, 68 a, and 68 b, inversion rollers for inverting theobverse and reverse surfaces of an image recording sheet; and 69 a and69 b, delivery rollers for delivering an image recording sheet outsidethe apparatus.

Reference numeral 70 denotes a fixing unit for fixing a toner image onan image recording sheet with heat and pressure. The fixing unit 70 hasa heat roller 71 (or 72) and a pressure roller 72 (or 71).

The arrangement of a writing unit using a laser beam in this manner isthe same as that shown in FIG. 14 which has already been described inthe first embodiment. In this case, the writing units 20Y, 20M, 20C, and20K also have the same arrangement.

The image printing apparatus according to the third embodiment will bedescribed in detail below with reference to the accompanying drawings.That is, the image printing apparatus according to the third embodimentis an image printing apparatus capable of printing images in a pluralityof colors, i.e., at least two colors. This embodiment will exemplify acolor image printing apparatus using toners of four colors, i.e., Y(Yellow), M (Magenta), C (Cyan), and K (blacK).

FIG. 11 shows a CPU 201 serving as a controller for controlling theoverall image printing apparatus, a writing unit circuit section 200Yfor Y, a writing unit circuit section 200M for M, a writing unit circuitsection 200C for C, a writing unit circuit section 200K for K, an LD260Y for performing exposure for Y, an LD 260M for performing exposurefor M, an LD 260C for performing exposure for C, an LD 260K forperforming exposure for K, an index sensor 266Y for Y, an index sensor266M for M, an index sensor 266C for C, and an index sensor 266K for K.

In this case, the writing unit circuit sections 200Y to 200K are circuitsections for the writing units 20Y to 20K for forming electrostaticlatent images on the surfaces of the photosensitive drums 10Y to 10K byscanning laser beams in accordance with pieces of image information ofthe respective colors.

Although FIG. 11 shows the detailed arrangement of the writing unitcircuit section 200Y, the remaining writing unit circuit sections 200M,200C, and 200K have the same arrangement.

Referring to FIG. 11, the writing unit circuit section 200Y has a dotclock adjusting section 210 which is a characteristic feature of thethird embodiment, an image processing section 220 for performing imageprocessing, and an LD driving section 230 for generating an LD drivingsignal in accordance with a dot clock on the basis of an imageprocessing result.

Assume that a color misregistration sensor 270 c is placed to detectpixel offsets at three or more positions in the main scanning directionand read predetermined test patterns of the respective color afterfixing.

The arrangement and operation of the dot clock adjusting section 210will be sequentially described below. Not that the dot clock adjustingsection 210 is a clock generating section in the claims. The dot clockadjusting section 210 is comprised of a clock generating section 211 forgenerating a fundamental clock and a frequency changing section 212capable of changing the frequency of the fundamental clock generated bythe clock generating section 211. In this case, the frequency changingsection 212 changes the leading or trailing edge timing or frequency ofa dot clock to a desired timing such as a given timing during scanningof one line or a timing corresponding to each image printing surface orimage printing color in order to change the position of a pixel printedin accordance with the dot clock. In the third embodiment, the frequencychanging section 212 is formed from a digital delay type dot clockadjusting device, and is comprised of (A), (B), (C), and (D) describedbelow.

(A) Delay Signal Generation:

A delay chain section 213 is a delay element group for obtaining aplurality of delay signals (delay signal group: in FIG. 11) havingslightly different phases from each other by delaying an input signal(the fundamental clock from the clock generating section 211).

In this case, in the delay chain section 213, delay elements arepreferably cascaded in the form of a chain to connect a sufficientnumber of delay elements to generate delay signals having slightlydifferent phases from each other over two fundamental clock periods.

Although the clock generating section 211 may be incorporated in each ofclock generating circuits for the respective colors, i.e., Y, M, C, andK, fundamental clocks may be distributed from the single clockgenerating section 211 to the clock generating circuits for therespective colors. As shown in FIG. 14, the index sensor 266Y detects areference position in scanning of a laser beam.

(B) Synchronization Detection:

A synchronization detecting section 214 is a detecting section whichreceives the index signal generated by the index sensor 266Y and detectsthe ordinal number of a delay signal (synchronization point), of a delaysignal group (in FIG. 11), which is synchronized with the index signal.This section outputs synchronization point information (in FIG. 11).

In this case, the synchronization detecting section 214 preferablyoutputs first synchronization point information SP1 indicating a delaysignal, of the delay signal group (in FIG. 11), which is synchronizedfirst with the index signal, and second synchronization pointinformation SP2 indicating a delay signal synchronized next with theindex signal.

Since a plurality of delay signals from the delay chain section 213 maygreatly vary in delay time due to the influences of temperature changesand the like, how many delay signals are included in a predeterminedunchanging time (the time interval between an index signal and anotherindex signal) is detected in advance in this manner.

(C) Selection Control:

A selection control section 215 obtains a synchronization correctionamount on the basis of synchronization point information (in FIG. 11)from the synchronization detecting section 214 and frequency offsetinformation (in FIG. 11) from the CPU 201, and outputs a select signal(in FIG. 11) indicating a delay signal having a specific phase which isto be selected from a delay signal group (in FIG. 11). The frequencyoffset information will be described later.

(D) Selection:

A selecting section 216 receives a select signal in FIG. 11) from theselection control section 215, and selects a delay signal having acorresponding phase from a delay signal group (in FIG. 11). Theselecting section 216 then generates a rectangular wave by setting theselected delay signal as leading and trailing edges, and outputs therectangular wave as a dot clock (in FIG. 11).

In this manner, the period of a dot clock is slightly increased ordecreased by setting a delay signal having a selected timing as leadingand trailing edges in generating a dot clock, thereby generating asignal whose pulse count representing the number of pulses generatedwithin a predetermined period of time is set to a predetermined count.

That is, the pulse count within the predetermined period of time is setto the predetermined count by sequentially selecting delay signals,whose phases (the positions or timings of the pulses of a dot clock) arefinely and gradually changed, within a predetermined period of timewithout changing the clock frequency itself instead of making adjustmentby finely adjusting the oscillation frequency of a fundamental clockoscillated and generated by the clock generating section 211.

In this manner, the period of a dot clock is slightly increased ordecreased by setting a delay signal having a selected timing as leadingand trailing edges in generating a dot clock. This can realize (a) and(b) described below.

(a) Generating a signal having a predetermined number of pulsesgenerated in a predetermined period of time makes the length of eachline in the main scanning direction uniform and makes the length betweenthe start and the end of each line constant.

(b) The timing of a dot clock is adjusted at three or more positions inthe main scanning direction by using pieces of error information at thethree or more positions in the main scanning direction, therebysuppressing pixel offsets among the respective colors includingintermediate portions.

Not only but also, which is unfeasible in the prior art, can be realizedby sequentially selecting delay signals, whose phases (the positions ortimings of the pulses of a dot clock) are finely and gradually changed,within a predetermined period of time without changing the clockfrequency itself instead of making adjustment by finely adjusting theoscillation frequency of a fundamental clock oscillated and generated bythe clock generating section 211.

<Principle (3) of Offset Detection for Dot Clock Adjustment>

The manner of offset detection in the third embodiment will be brieflydescribed below with reference to the flow chart of FIG. 12 and theschematic view of FIG. 3.

At the time of offset detection, the writing unit circuit section 200generates images with predetermined patterns stored in a ROM 221 at atotal of three or more positions, i.e., the start position, endposition, and intermediate position on the intermediate transfer memberbelt 40 in the main scanning direction (S1 in FIG. 12). FIG. 3 shows acase wherein patterns in the form of the Katakana character “

” are printed as predetermined patterns at three positions in the mainscanning direction.

Assume that although the patterns indicated by the solid lines areactually printed on the image recording sheet, the patterns indicated bythe broken lines were expected to be printed.

In this case, offsets dx1, dx2, and dx3 have occurred in the mainscanning direction due to the aberrations of the writing unit and therespective optical systems and the like. When these patterns are read bythe color misregistration sensor 270 c disposed at a position where itcan read the patterns on the image recording sheet while theintermediate transfer member belt 40 is moved in the sub-scanningdirection (S2 in FIG. 12), a distance Y1′ from the horizontal line tothe oblique line of the pattern in the form of the Katakana character “

” includes an offset dy1, and distances Y2′ and Y3′ respectively includeoffsets dy2 and dy3.

Letting è be the angle defined by the horizontal and oblique lines,dx1=dy1/tan è. In addition, the offset dy1 can also be obtained from themoving speed of the image carrier in the sub-scanning direction and thedifference between the times when the horizontal and oblique lines areread.

Likewise, letting è be the angle defined by the horizontal and obliquelines, dx2=dy2/tan è. Furthermore, the offset dy2 can be obtained fromthe moving speed of the image carrier in the sub-scanning direction andthe difference between times when the horizontal and oblique line areread.

Likewise, letting è be the angle defined by the horizontal and obliquelines, dx3=dy3/tan è. Furthermore, the offset dy3 can be obtained fromthe moving speed of the image carrier in the sub-scanning direction andthe difference between times when the horizontal and oblique line areread.

In the above manner, the positions of the test patterns (offset states)are calculated by the CPU 201 (S3 in FIG. 12).

For the remaining colors, in the same manner as described above, testpatterns are printed (S4 in FIG. 12), the test patterns are read by thecolor misregistration sensor 270 c (S5 in FIG. 12), and the positions(offset states) of the test patters are calculated (S6 in FIG. 12). TheCPU 201 then compares the positions (offset states) of the test patternsin the respective colors to calculate pixel offset states for therespective colors (S7 in FIG. 12).

Note that the CPU 201 may calculate pixel offset states for therespective colors by comparing the positions (offset states) of the testpatters of the respective colors with the reference distances based onthe test patterns.

Therefore, by printing and reading such predetermined patterns of therespective colors, i.e., Y, M, C, and K, at the same position in thesub-scanning direction and three or more positions in the main scanningdirection, e.g., at the start side in the main scanning direction, theend side in the main scanning direction, and an intermediate positiontherebetween, the CPU 201 can detect offset states (frequency offsetinformation) associated with the expansion/contraction of an image inthe main scanning direction and pixel offsets in an intermediateportion.

The CPU 201 performs the above detection processing for the respectivecolors in this manner, and supplies frequency offset information (inFIG. 11) to the writing units 20Y to 20K.

Likewise, the CPU 201 can obtain image leading end offset informationassociated with the start position of an image in the main scanningdirection by detecting the pattern in the form of the Katakana character“

” on the start end side in the main scanning direction, and can supplythe image leading end offset information to the writing unit.

In the above manner, in usual image printing operation, on the basis offrequency offset information representing pixel offset states for therespective colors, the dot clock adjusting section 210 generates dotclocks for eliminating the pixel offset states for the respectivecolors, thereby printing an image (S8 in FIG. 12)

<Operation for Color Misregistration Adjustment of Color Image PrintingApparatus>

As in the above correction, and more specifically, as described in thefirst embodiment with reference to FIGS. 4 to 6, offsets in the mainscanning direction can be adjusted by using the digital delay type dotclock adjusting device. Setting the length of one main-scanning line ofone of Y, M, C, and K as a reference length and matching the length ofone main-scanning line of each of the remaining colors with thereference length to align the leading end positions of the respectivelines can suppress color misregistration in printing a color image.

That is, the leading end position and length of one line of a givencolor in the main scanning direction are measured by the technique shownin FIG. 3, and the dot clock adjusting section is so operated as tomatch the leading positions and lengths of lines of the remaining colorsin the main scanning direction with those of the line of the givencolor. This allows the images of the respective colors to coincide witheach other, thus eliminating color misregistration in the main scanningdirection which is caused by various factors.

In this case, proper correction can be made for color misregistrationand the length in the main scanning direction by performing controlbased on the characteristics shown in FIGS. 13A to 13E in accordancewith the color misregistration that has currently occurred.

The use of the digital delay type dot clock adjusting section in theabove manner can adjust an offset between the start and the end in themain scanning direction. A pixel offset for each color can be suppressedby adjusting the timing of a dot clock at three or more positions in themain scanning direction using pieces of error information at three ormore positions in the main scanning direction for each color.

The above specific example has exemplified the case of the test patternsat the three positions. However, increasing the number of test patternswill suppress obverse/reverse pixel offsets more finely. If, forexample, test patterns are printed at four positions, obverse/reversepixel offset suppression can be done in areas on the left endportion/near the center/on the right end portion. That is, offsetcorrection can be done at three positions by using test patterns at fourpositions, thereby obtaining a better result.

This embodiment uses the digital circuit arrangement having no analogfeedback circuit such as a PLL. This circuit and other digital circuitportions can therefore be Integrated into one chip. This makes itpossible to perform obverse/reverse pixel offset elimination with higherprecision.

Letting L be the number of pixels per line in the main scanningdirection which are used for image printing, L1 to Ln−1 be the numbersof pixels in the respective ranges divided by n test patterns (n isequal to or more than 3) and N1 to Nn−1 be the offset amounts calculatedbetween the three or more test patterns, it is preferable that an offsetcorrection amount Nj/Lj (where 1 j n) between each pair of test patternsbe evenly distributed to the respective pixels between each pair of testpatterns so as to be reflected in the selection of a delay count for adot clock.

In this manner, error information is detected at three or more positionsin the main scanning direction and is evenly distributed to therespective pixels between each pair of test patterns so as to bereflected in the selection of a delay count for a dot clock. This canalign the starts and the ends of the respective lines and properlyperform pixel offset adjustment in a halfway portion.

In the third embodiment described above, the CPU 201 is externallyprovided for the dot clock adjusting section. However, the dot clockadjusting section may incorporate controllers such as CPU and tables.

As described above in detail above, according to the third embodiment,when a dot clock is generated for each of a plurality of colors used toprint an image, and exposure for each color is to be performed on thebasis of the dot clock, a plurality of delay clocks are generated byfinely delaying a fundamental clock, and the manner of selection for theplurality of delay clocks is changed to change the leading edge timingor trailing edge timing of the dot clock generated for a color for whichan offset should be adjusted. In this state, an image printed in a givencolor (or the desired value of image data of a test pattern) is set as areference, and test patterns are printed at three or more positions inthe main scanning direction with respect to image offsets of theremaining colors, thereby detecting error information associated withcolor misregistration. The selection of a plurality of delay clocks inthe above digital delay type dot clock adjusting device is controlled onthe basis of the error information, thereby correcting the offset. Sincethis embodiment uses the digital circuit arrangement having no analogfeedback circuit such as a PLL, the circuit and other digital circuitportions can be integrated into one chip. This makes it possible toperform color misregistration elimination with higher precision. Inaddition, in the third embodiment, offset correction is executed upondetection of error information at three or more positions in the mainscanning direction. This can align the starts and the ends of therespective lines and adjust a pixel offset in an intermediate portion.

According to the third embodiment, in the digital delay type dot clockadjusting device, the delay chain section generates a plurality of delayclocks by finely delaying a fundamental clock, and the synchronizationdetecting section detects synchronization information. The selectioncontrol section then generates a select signal from the synchronizationinformation and error information. The selecting section selects a delayclock corresponding to the select signal from a plurality of delaysignals and outputs it as a dot clock. Since the digital delay type dotclock adjusting device has a digital circuit arrangement having noanalog feedback circuit such as a PLL, the circuit and other digitalcircuit portions can be integrated into one chip. This makes it possibleto perform high-precision color misregistration elimination.

The third embodiment includes digital delay type dot clock adjustingdevices as independent circuits for the respective colors for whichcolor misregistration should be performed. This makes it possible toperform high-precision color misregistration elimination.

In the third embodiment, the offset detecting section, digital delaytype dot clock adjusting device, and controller are formed from digitalcircuits. These sections can perform color misregistration eliminationwith high precision in a state wherein they are suitable for integrationinto one chip.

In the third embodiment, since the image printing apparatus operatesunder the control of a controller such as a CPU externally provided forthe digital delay type dot clock adjusting device, color misregistrationelimination can be performed with high precision.

In the third embodiment, since the image printing apparatus operatesunder the control of a controller such as a CPU incorporated in thedigital delay type dot clock adjusting device, color misregistrationelimination can be done with high precision by the digital delay typedot clock adjusting device in a state wherein the device is suitable forintegration into one chip.

The third embodiment described above is directed to colormisregistration correction. If, however, a pattern in the form of theKatakana character “

” to be printed on the start side in the main scanning direction isplaced as near the end portion at the start position in the mainscanning direction as possible, and image leading end offset informationassociated with the start position of an image in the main scanningdirection is supplied to the selection control section 215 in thewriting unit circuit section 200, the start positions of the respectivelines in the main scanning direction can be aligned.

According to the above description of the third embodiment, colormisregistration is detected by the color misregistration sensor 270 c,and an offset is set. In addition to this operation, the CPU 201 mayanalyze the image data obtained by making a scanner (not shown) read animage recording sheet on which test patterns are printed, and set anoffset. Alternatively, an operator may measure an image recording sheeton which test patterns are printed, and set an offset obtained from themeasurement result with respect to the CPU 201 with an operating section(not shown).

Other Embodiments

In the first, second, and third embodiments, the electrophotographicimage printing apparatuses using laser beams have been described.However, the present invention is not limited to this For example, eachembodiment of the present invention can be applied to various kinds ofimage forming apparatuses such as a laser imager for exposing a sheet ofphotographic paper to a laser beam and an inkjet printer whichdischarges ink from a head, and good results can be obtained.

1. An image printing apparatus comprising: clock generating means forgenerating a dot clock as a basis of each pixel forming an image; imageprinting means for printing a one-line image in a main scanningdirection in accordance with image data with reference to the dot clockoutput from said clock generating means, and printing a one-page imageby repeating in the sub-scanning direction one-line image printingperformed in the main scanning direction; and control means forcontrolling image printing in the main scanning direction such thatdistances between test patterns in reference images which are arrangedat least three positions in the main scanning direction coincide withdistances between test images printed in accordance with test image datacorresponding to the reference images.
 2. An apparatus according toclaim 1, wherein said control means comprises a clock control sectionwhich changeably controls a frequency of the dot clock during scanningof one line.
 3. An apparatus according to claim 2, wherein said clockgenerating section comprises a fundamental clock generating sectionwhich generates a fundamental clock having a predetermined frequency,and a frequency changing section which can change a frequency of thefundamental clock generated by said fundamental clock generatingsection, and wherein said clock control section changes a frequency of afundamental clock output from said frequency changing section duringscanning of one line in the main scanning direction.
 4. An apparatusaccording to claim 3, wherein said frequency changing section has adelay line which generates delay clocks by digitally delaying thefundamental clock, and said clock control section changes the frequencyof the dot clock during scanning of one line in the main scanningdirection by selecting a predetermined delay clock from the delay line.5. An apparatus according to claim 2, wherein said image printingapparatus prints images on two surfaces of an image recording sheet, andsaid clock control section changes the frequency of the dot clock whenan image is printed on one surface of the image recording sheet orimages are printed on the two surfaces.
 6. An apparatus according toclaim 2, wherein said image printing apparatus prints an image bysuperimposing an image formed in a first color and an image formed in asecond color different from the first color, and said clock controlsection changes the frequency of the dot clock when one or both ofimages in the first and second colors are to be printed.